1. Field of the Invention
The invention is generally related to integrated circuits and more specifically to integrated circuit processing, design, and debugging.
2. Description of the Related Art
Prior art fiducials have typically been produced with a single layer or multiple layers of metals deposited on a semiconductor substrate in a characteristic shape, such as a cross or plus-sign (‘+’) shape, or some similar but recognizable shape. By using a recognizable shape, these fiducials have been constrained to be large patterns which prove distinctive when viewed by the people who use them for navigating on a semiconductor integrated circuit. A fiducial in the prior art would often consume a square-shaped space on the integrated circuit 150 μm on a side, which could only be used for the fiducial, not for any active circuitry. As a result, valuable resources on the integrated circuit would be unavailable in that region.
FIG. 1 illustrates one prior art scheme for placement of fiducials. Package 110 contains integrated circuit 130. Package 110 also have four package fiducials 120 located on the outside of package 110, which are used by someone who needs to locate a specific portion of integrated circuit 130. After locating and aligning to a first package fiducial 120, a portion of the package 110 may be removed to expose integrated circuit 130. Each of four fiducials 140 are incorporated into integrated circuit 130. Upon aligning to a first fiducial 140, a person may then navigate over the integrated circuit 130 by looking at a layout diagram of integrated circuit 130 which shows the location of the fiducials 140 relative to the circuitry incorporated in integrated circuit 130.
As will be appreciated, positioning the fiducials such as fiducials 140 proves difficult due to constraints on available space on integrated circuit 130. In the case of a fiducial consuming a square of space 150 μm on a side, four such squares must be reserved in the area available on integrated circuit 130, and no other signals may be routed in those reserved areas.
Furthermore, even in situations in which automated alignment equipment is used, such equipment must use an optical system (a system for resolving optical details of an image, typically using reflected light) for locating the fiducials. Whether human, mechanical, or some combination of human and mechanical, the optical systems are limited by their inability to resolve images below a certain size (length or area) threshold on semiconductor devices due to the wavelength of light used. This limitation is particularly constraining in infrared systems which often cannot resolve features smaller than 1-2 μms, and therefore cannot reliably navigate to less than 1-2 μms from a desired location on the semiconductor. This limitation leads to a limitation on the size of fiducials used for alignment when using optical alignment systems, thus leading to the 150 μm length of prior art fiducials. It will be appreciated that even though an optical system may be capable of resolving features much smaller than the overall size of a fiducial, that the need for a distinctive shape of the fiducial leads to a fiducial much larger than the size of the smallest feature an optical system may resolve.